ECG STUDY GUIDE
1. Explain key definitions related to understanding the electrical conductivity of
the myocardium
• The electrical conductivity of the myocardium relies on specialized tissues
and cellular events:
• Cardiac Conduction System
• SA node (Sinoatrial node): This is the natural pacemaker, located in
the Right Atrium at the junction of the Superior Vena Cava. It
typically
generates 60–100 beats per minute (BPM).
• AV node (Atrioventricular node): Located in the Right Atrium at the
posterior-inferior atrial septum. Its primary function is to slow
electrical conduction, which allows time for the ventricles to fill. It is
capable of
pacing at 40–60 BPM.
• Bundle of His: This pathway speeds up electrical conduction.
• Purkinje Fibers: These fibers are found throughout the ventricles and,
along with the Bundle of His, can serve as intrinsic pacemakers at 20–
40
BPM.
• Cardiac Electrical Events
• Depolarization: This is the fundamental electrical event of the heart.
It is the loss of the difference in charge across the cardiac cell
membrane
due to the influx of Na+ ions into the cell. Depolarization initiates the
contraction of the cardiac muscle.
▪ In the Cardiac Action Potential (AP), rapid depolarization is
known as Phase 0 and corresponds to the beginning of the QRS
complex, involving sodium influx.
• Repolarization: This occurs when the cardiac cell returns to its 'resting
state'. It involves the efflux of Na+ back out of the cell and results in the
relaxation of the cardiac muscle. Repolarization must occur before the
cell can be depolarized again.
▪ In the AP, Phase 3 corresponds to the T wave and involves the
rapid efflux of K+, leading to accelerated repolarization.
• Cardiac Action Potential (AP): This is a graphic demonstration
showing the change in voltage of a single cardiac cell over time when
the cell membrane is destabilized by the influx of Na+/Ca+ ions. The
12 Lead ECG represents the sum total of all AP’s of the heart over
time.
2. Identify where to place the 12 leads of a patients' chest
• The sources identify the existence of Limb Leads (Frontal Plane View) and
Precordial / Chest Leads.
• The importance of accuracy, noting that if placement is off by one intercostal
space (ICS), it can change the QRS shape and lead to missing ST elevation. A
standard 12-lead ECG also does not capture the Posterior Left Ventricular
(LV) wall and Right Ventricle (RV).
3. Describe basic and advanced concepts of reading an ECG
, • The 12 Lead ECG provides information on the orientation of the heart in the
chest, the presence or absence of conduction defects, the electrical effects of
medications and electrolytes, and the mass of the cardiac muscle.
• Basic ECG Measurement and Calibration
• An ECG is recorded on specialized paper:
• Vertical (Height): Measures voltage in millimeters (mm). 1 small box
= 1 mm elevation; 1 LARGE box = 5 mm elevation.
• Horizontal (Width): Measures time (velocity) in milliseconds (ms).
▪ 1 small box = 0.04 seconds.
▪ 1 LARGE box = 0.20 seconds.
▪ 5 LARGE boxes = 1 second.
• Calibration: A standard calibration box is typically 10 mm high and
0.20 seconds wide.
• Systematic Analysis: A comprehensive analysis of a 12-lead ECG
includes checking the following components:
• Rate (atrial and ventricular).
• Rhythm (atrial and ventricular).
• Intervals (PR interval, QRS duration, QT interval).
• Waveforms (P waves, Q waves, R waves (progression), T waves,
U waves).
• J point.
• ST segment (elevation, depression).
• Axis.
• Myocardial ischemia, injury, or infarction.
• Effects of medications and electrolyte imbalances.
• Advanced Concepts: Q Waves and Axis Deviation
• Q Wave Significance: Small, insignificant, non-pathologic Q waves
represent the depolarization of the septum and are commonly found
in
Leads I, aVL, and V6.
• A Significant / Pathologic Q wave is an indication of dead
myocardial tissue. The criteria are:
▪ Wider than 1 small square (i.e., $>0.04$ seconds).
▪ Height equal to or greater than 1/3 the height of the R wave in
the same QRS complex.
▪ They develop within hours and are permanent.
• Axis Determination: The cardiac axis relates to the position of the heart in the
chest cavity and the flow of electrical activity through the heart. It is measured
using the Hexaxial Reference Plane.
• Rule of Thumbs (Easy Axis Determination):
• If the "thumbs" (referring to the net QRS deflection in Leads I and
aVF) are heading "RIGHT" for each other, it suggests Right Axis
Deviation.
• If the "thumbs have “LEFT” each other, it suggests Left Axis
Deviation.
• Clinical Significance: Axis deviations can indicate conditions such as
Left Bundle Branch Block, Left Ventricular Hypertrophy,
1. Explain key definitions related to understanding the electrical conductivity of
the myocardium
• The electrical conductivity of the myocardium relies on specialized tissues
and cellular events:
• Cardiac Conduction System
• SA node (Sinoatrial node): This is the natural pacemaker, located in
the Right Atrium at the junction of the Superior Vena Cava. It
typically
generates 60–100 beats per minute (BPM).
• AV node (Atrioventricular node): Located in the Right Atrium at the
posterior-inferior atrial septum. Its primary function is to slow
electrical conduction, which allows time for the ventricles to fill. It is
capable of
pacing at 40–60 BPM.
• Bundle of His: This pathway speeds up electrical conduction.
• Purkinje Fibers: These fibers are found throughout the ventricles and,
along with the Bundle of His, can serve as intrinsic pacemakers at 20–
40
BPM.
• Cardiac Electrical Events
• Depolarization: This is the fundamental electrical event of the heart.
It is the loss of the difference in charge across the cardiac cell
membrane
due to the influx of Na+ ions into the cell. Depolarization initiates the
contraction of the cardiac muscle.
▪ In the Cardiac Action Potential (AP), rapid depolarization is
known as Phase 0 and corresponds to the beginning of the QRS
complex, involving sodium influx.
• Repolarization: This occurs when the cardiac cell returns to its 'resting
state'. It involves the efflux of Na+ back out of the cell and results in the
relaxation of the cardiac muscle. Repolarization must occur before the
cell can be depolarized again.
▪ In the AP, Phase 3 corresponds to the T wave and involves the
rapid efflux of K+, leading to accelerated repolarization.
• Cardiac Action Potential (AP): This is a graphic demonstration
showing the change in voltage of a single cardiac cell over time when
the cell membrane is destabilized by the influx of Na+/Ca+ ions. The
12 Lead ECG represents the sum total of all AP’s of the heart over
time.
2. Identify where to place the 12 leads of a patients' chest
• The sources identify the existence of Limb Leads (Frontal Plane View) and
Precordial / Chest Leads.
• The importance of accuracy, noting that if placement is off by one intercostal
space (ICS), it can change the QRS shape and lead to missing ST elevation. A
standard 12-lead ECG also does not capture the Posterior Left Ventricular
(LV) wall and Right Ventricle (RV).
3. Describe basic and advanced concepts of reading an ECG
, • The 12 Lead ECG provides information on the orientation of the heart in the
chest, the presence or absence of conduction defects, the electrical effects of
medications and electrolytes, and the mass of the cardiac muscle.
• Basic ECG Measurement and Calibration
• An ECG is recorded on specialized paper:
• Vertical (Height): Measures voltage in millimeters (mm). 1 small box
= 1 mm elevation; 1 LARGE box = 5 mm elevation.
• Horizontal (Width): Measures time (velocity) in milliseconds (ms).
▪ 1 small box = 0.04 seconds.
▪ 1 LARGE box = 0.20 seconds.
▪ 5 LARGE boxes = 1 second.
• Calibration: A standard calibration box is typically 10 mm high and
0.20 seconds wide.
• Systematic Analysis: A comprehensive analysis of a 12-lead ECG
includes checking the following components:
• Rate (atrial and ventricular).
• Rhythm (atrial and ventricular).
• Intervals (PR interval, QRS duration, QT interval).
• Waveforms (P waves, Q waves, R waves (progression), T waves,
U waves).
• J point.
• ST segment (elevation, depression).
• Axis.
• Myocardial ischemia, injury, or infarction.
• Effects of medications and electrolyte imbalances.
• Advanced Concepts: Q Waves and Axis Deviation
• Q Wave Significance: Small, insignificant, non-pathologic Q waves
represent the depolarization of the septum and are commonly found
in
Leads I, aVL, and V6.
• A Significant / Pathologic Q wave is an indication of dead
myocardial tissue. The criteria are:
▪ Wider than 1 small square (i.e., $>0.04$ seconds).
▪ Height equal to or greater than 1/3 the height of the R wave in
the same QRS complex.
▪ They develop within hours and are permanent.
• Axis Determination: The cardiac axis relates to the position of the heart in the
chest cavity and the flow of electrical activity through the heart. It is measured
using the Hexaxial Reference Plane.
• Rule of Thumbs (Easy Axis Determination):
• If the "thumbs" (referring to the net QRS deflection in Leads I and
aVF) are heading "RIGHT" for each other, it suggests Right Axis
Deviation.
• If the "thumbs have “LEFT” each other, it suggests Left Axis
Deviation.
• Clinical Significance: Axis deviations can indicate conditions such as
Left Bundle Branch Block, Left Ventricular Hypertrophy,
